Rat IL3 is a pleiotrophic cytokine produced by activated T cells. IL3 functions via specific cell surface receptors to activate hematopoiesis by controlling the production, differentiation, and proliferation of granulocytes and monocytes. IL3 also affects the activity of mature mast cells, eosinophils, basophils, and macrophages. The high affinity IL3 receptor is composed of an alpha and beta subunit, where the IL3R alpha is a member of the cytokine receptor super family and binds to IL3 with low affinity. The beta subunit does not bind any cytokine but binds to the alpha subunit of the IL3, IL5 and GM-CSF receptors with high affinity. The receptors for IL3 can be found in bone marrow, macrophages, mast cells, megakaryocytes, and other similar cells.